Electronic control circuits



April 20, 1954 w. B. HILLS ET AL 2,676,297

ELECTRONIC CONTROL CIRCUITS Filed June 21; 1952 Figl.

JPHASE (aOCYCLE POWER INPUT 28 L2 Inventors: "7 William B.Hi|ls, Carter Sinclair, b WW Their-Attorney.

Patented Apr. 20, 1954 William Hills and Carter Sinclair; schenec tady, N.-Y., assignor's to General Electric- Coinparty; a". corporation of New York eppu eaabnlune 21, 1952, Serial No. 294,906

Ourinvention relates to electronic" control cir= cuits and more particularly to" frequency amiphaseconverting systems embodying electronic control means" operating in response toth'e firing of one of the main" electric discharge devices in the polyphase" supplycircuit thereof for changing without interruption the load current flow from" one" value to another at a selected point in the lowfrequencysingle-phase half cycle supplied by such system:

Frequency and phase converting systen'isare employed ior'supplying" a single phase resistance welding load from a polyph'a'se source'ofsu'pp'ly because of certain a'clv'antages'such as uniformly loadingthe pol'ypirase suppiy with a single phase load 'reducing'thereactanceof the load due to'the low' frequency employedj and preventing sputtering and burning at the" weld because of the increase in current now from an initially low value-- to the" welding value; Such converting systems arealso advantageouslyemployed vfor half" cycle resistance" spot welding because it is possible to adfl'iust the duration of the" current pulse by adjustingthe changein frequency se-- cured thereby. When welding with such half cycle welders,- it desirable to start each new welding operation with weldingcurrent andvoltage of opposite polarities in order to avoid" the direct current" saturating effecton transformer which would" otherwise repeated applications of voltages of" the same polarity. Such operation with immediately following current pulses of alternate polarity is reierred toan anti-polar starting.

There are, however, certain welding operations that require a weld period immediately followed by a heat treating period. Thus in spot welding parts-f'ormedof certain aluminum alloys, shrinkage cracks will occur inth'e weld'after its formation if it is-immediately' subjected tothe quenching action resultingfrom the rapid extraction of heat from the weld by the water cooled electrocleswhich-engage the work'parts at the weld. Subsequently heating of the weld while still clamped between the electrodes will not eliminate these cracks, although it may restore some hardness to the weldwhich may beleft in a soft condition due to therapid cooling immedi ately follbwingt'hewelding operation.

This quenching action may be'eliminated by changing themagnit'urle of current flow from a welding value to a heat-treating value during the low frequencyhalfcycle supplied bythe converter: Since, however; the welding time for the high current values" usually employed is quite the welding result from critical for these aluminum alloys; it is necessary to have this changeover from Welding to heat treating current flow occur uniformly" at the same selected point in" the half cycle of applied current. Specifically this changeover in current value must take placebetween the firing" of a} oer tain two electric discharge devices employed: in the electronic converter for enerating the we frequency pattern.

In a three-phase to singlephase' converting tem, current for each low frequency halficycle' is supplied in phasesequencefrointhe A, and 0 phases of the source. The magnitude ofciil: rent now is determined by controlling thepoints' in the half cycles of each phase voltageof the" three phase source at which current now is inttiated and the time of current flow is deter mined by controlling the number of phasese quenc'esused in generating the low frequency-half cycle of current flow.

In electronic converters; currentflow is" controlled by electric discharge'devices each having an anode, a cathode and at controlelernentwhoee excitation controls the now or current between its anode-and cathode; The anode -catho'de cii: cults of these electric discharge devices" interconnect the'polyphase supply conductors withthe single phase load cohductors which are-connected to the welding electrodes; The point" in each half cycle of positive anode voltage or the else tric discharge devices at which current'fiow" is initiated is determinedby phase shift circuitswhich control the tiin'es inthe positivehalf ey cles of anode voltages at which voltage is ap: pliedto their control elements to" initiate conduction between their anodes and cathodes.

The static phase shift circuits usually" em ployed' comprise a bridge,- one arm of which is formed by one or more series connected adjust able resistors whose settings' determine the amount of phase shift-L To provide independent adjustment of the welding current and heat treating current during a half cycle or current flow; at least two such adjustable" resistbr must be employed 'a-nol thcheat treating adjustable-n c sistor must-be shortened from the circuit when the other adjustable resistor isi'nclu'cled" in the circuit for setting" the flower weldingcurreht'at a desired value. The mosteffective' W'a'y of' aecom'pli'shing this is by" empmying the contacts of an electromagnetic relay since itis' notpossible to obtain as low a resistance short circuit of the heat treating adjustable resistor through the electrodes of an electfic' discharge device; Furthermore the different phase voltages for 3 each electric discharge device and the employment of a single control voltage for their controlled operation interpose complications and cost factors which are to be avoided.

Ihe use of such an electromagnetic relay, however, imposes its own complications since it must operate to change the setting of the phase shift circuit it controls at a time not within the heat setting range of welding current adjustment for each phase voltage or else sufficient variation in welding current flow will occur to produce defecti'v'e welds.

It is an object of our invention to provide an electronic frequency and phase converting system in which an electromagnetic relay is employed for consistently changing the current setting of the converter from a welding value to a heat treating value and in which the effect of variations in operating time of such a relay is minimized by synchronizing its energization with the initiation of current flow in one of the phase pairs of electric discharge devices forming part of the converter.

It is also an object of our invention to provide for anti-polar starting of a half cycle frequency and phase converting system under the control of a timing means which is equally effective for securing the same times of operation irrespective of the 180 electrical degree difference in the starting times of positive and negative phase voltage sequences.

It is another object of our invention to provide means for supplying a substantially uniform voltage signal to the control element of an auxiliary electric discharge device in response to the occurrence of momentary current values of widely varying magnitudes such as flow in the firing circuits of arc discharge devices of the immersion igniter type which are usually employed in converters of the type under consideration.

It is also an object of our invention to provide a control circuit for supplying energization in response to two non-concurrent momentary signal voltages, one of which conditions the circuit for operation in response to the other thereof.

Further objects of our invention will become apparent from a consideration of the embodiment thereof illustrated in the accompanying drawing and described below.

Fig. 1 of the accompanying drawing is a sim- I plified diagrammatic representation of a half cycle frequency and phase converter which embodies our invention.

, Fig. 2 is a voltage diagram to which reference will be made in describing the operation of the system illustrated in Fig. 2.

In the half cycle frequency and phase converter shown in the accompanying drawing provision is made for anti-polar starting under control of the same timing means which times out the total time period one-half cycle later for voltages of one polarity than for voltages of the other polarity in order to compensate for the one-half cycle difference in the starting times of these voltages of opposite polarity. After the lapse of the time period for welding current flow, a momentary signal voltage is applied by the timer to the control element of an arm discharge devise which then completes its anode-cathode connection through a current limiting circuit across direct current supply conductors and also connects the cathode of a second arc discharge device to one of these direct current supply conductors. Thereafter a second momentary signal voltage is introduced into the control element circuit of the second arc discharge device in response to current flow in the firing circuit of one of the phase pairs of the main electric discharge devices of the converter. This renders the second arc discharge device conductive to energize the operating winding of a relay which is connected in series with its anode-cathode circuit across the current limiting circuit and consequently across the direct current supply conductors. Operation of the relay then changes the setting of the phase shift circuits employed for controlling the times in positive half cycles of anode voltage when the main electric dicharge devices of the converter become conducting. The converter then operates for the heat treating period at reduced current flow which is often referred to as tailing current. After the lapse of the preset tailing current flow period, the timer operates to interrupt the flow of current by applying hold-off voltages to the control elements of the main electric discharge devices of the converter.

Response of the above-referred to second electric discharge device to the firing current of one of the selected phase pairs of main electric discharge devices of the converter is secured by associating a current transformer with eachof the selected phase pair of main electric discharge devices and connecting the primary of each current transformer in a different one of these firing circuits of this pair of main electric discharge devices. The secondaries of the current transformers are connected in parallel with one another and in series with the control element circuit of the arc discharge device. Since the firing currents of the main electric discharge device vary widely in magnitude, it is necessary to protect the current transformers and the arc discharge device from over voltage conditions which might occur at the highest firing current values. This protection is obtained by using current transformers having decided drooping characteristics, that is a large regulation of voltage with change in current, and connecting across their secondaries a voltage limiting means such as a resistor having a hyperbolic resistance current characteristic producing a substantially constant voltage across its terminals. A suitable material for this purpose is described and claimed in United States Letters Patent 1,822,742-Karl l3. McEachron, granted September 8, 1931.

The frequency and phase converting system shown in Fig. l of the drawing comprises a power unit illustrated at the top of the figure, an exciter therefor illustrated at the mid-portion of the figure, and a sequencer illustrated at the bottom of the figure. This sequencer embodies a synchronising circuit, shown at the lower righthand portion of the figure, which includes an electrc-magnetic relay whose energization is controlled in response to a momentary signal voltage received from the sequencer and a subsequently occurring momentary signal voltage received from the power unit. The operation of this relay controls the settings of the phase shift circuits of the exciter.

The converter per se consists of the power unit and its exciter which together may be considered as a controlling means interconnecting the polyphase supply conductors with the single phase load conductors for current flow of one polarity or the other to the load conductors. Thi controlling means due to the organization of its exciter has a plurality of settings determinative of the points in the half cycles of voltage of the.

aerator supply-conductors at which current flows from the" supply conductors to the load conductors. The sequencer initiates operation of the controlling means at one setting thereof and after a predetermined number of periods of conduction of the controlling means at said one setting, the above referred to relay by its operation changes the setting of the controlling means in response to initiation of current conduction in a particu-- lar one of the phase circuits of the controlling means.

In the embodiment illustrated, the power unit supplies current from the three phase supply conductors Li, L2 and L3 to the single phase load conductors connected to the welding electrodes l of a resistance welding machine. As shown these electrodes I are connected to the secondary 2' of a welding transformer 3 having three pri-- mary windings i, 5 and 6. Each of these threeprimary windings of the welding transformer is connected across diiferent pairs oi the three phase supply conductors tl rough three pairs or reversely connected main electric discharge devices With each pair thereof being connected in serie circuit with a different one of the three primary windings of the welding transformer; Thus primary winding l of the transformer is connected across the A phase of the supply between supply conductors Li and L2 through a pair of main electric discharge devices i and 8, primary winding of the transformer is connected across the 33 phase of the supply between supply" conductors L2" and L3 through a pair of main electric discharge devices 3 it, and primary winding 6 is connected across the C phase of the supply between supply conductors L3 and Li through a pair of main electric discharge devices ii and 12.

Each of the main electric discharge devices 'l'to 52 inclusive may be of any thetypes well known in the art but as illustrated each comprises as anode 3, a mercury pool cathode and an immersion igniter type control element I 5. The ignite-r elements it may be formed of a high resistance material such as a carbide and silicon carbide and'are arranged in the electric discharge devices so that their tips are immersed in the mercury cathode pools of these devices. The electrodes of each of these devices are enclosed in an envelope which preferably'contains, as indicated by the dot, an ioniza'ble medium such as a'gas or vapor.

In order to prevent crowding of the drawing Withreference numerals only the electrodes of elect 'ic' discharge device have been identified by numerals. It is to be understood, however; that like electrodes of the remaining electric discharge devices 8- to it? inclusive will be identified by the same reference numerals, since the like symbolism employed in each case fully identifies like elements;

Each of the electric discharge devices "I to l2 inclusive is provided with an anode firing circuit it to 21' inclusive. Each of these circuits'includes its own firing electric discharge device 22 to" 2'! inclusive. Each of these firing electric discharge devices is provided with an anode 28, a cathode 2 ii and a control element 30. These reference numerals have been applied only to the electrodes of the firing electric discharge" device 22'but it isto be understood that like electrodes of the remaining firing electric discharge devices will be identified by referring to the same reference numerals; The electrodes of each ofthe firing electric discharge devices is enclosed" within an mixture of boron envelope which preferably contains; as indicated by the dot, an ionizable medium such as a gas or vapor.

The firing circuits of each of the main electric discharge devices are completed through the anode-cathode circuit of its associated firing electric discharge device. When current flows in this firing circuit of a main electric discharge device; ionization is produced in this electric discharge device which initiates its conduction. Once conduc tion is initiated in the main electric discharge device, current fi'ow'either ceases or is reduced to a very low value in the firing circuit including the firing electric discharge device associated with the main electric discharge device. Thus current how in the firing circuit and through the firing electric discharge device is only a momentary occurrence.

Each of the firing electric discharge devices 22 to 21 inclusive has its own control element circuit connecting its control element and cathode. In the arrangements illustrated, each control element circuit is provided with twobranches, one ofwhich may be referred to as the heat control branch and the other of which may be referred to as the timing branch. The heat control branch of'each of the control element circuits of each of the firing electric discharge de vices has been respectively identified in the drawing by reference numerals 35 to ll-ii inclu sive. The timing branch of each of these control element circuits has been respectively identified by reference numerals 37 to 42 inclusive.

The heat control branches of each of the trol' element circuits of the firing electric discharge devices 2 2' to 27 inclusive are completed respectively through voltage absorbing resistors 3 to as inclusive and .the secondary windings it to 5% inclusive of the phase shift circuit transformers 55- to 5-"! inclusive of the exciter. The timing branches of the control element circuits of the firing electric discharge devices are completed respectively through diodes 551 to 63 inclusive and the secondary windings 84': to $9 inclusi-ve of trigger circuit transformers ill to "it inclu- 51% and sources of bias voltage "it to EH inclu sive. It will be-notedthat the diodes 58 to 53 inelusive are connectedin respective timing branches of the control element circuits with anodes connected toward the control elements of their associated firing electric discharge devices 22 to 2"? inclusive. The arrangement in each control element circuit of the firing electric discharge devices is such that the more nega tive of the control voltages in the two branches is eii'ective for controlling the conductivities of the firing electric discharge devices. When no voltages are induced in transformer windings 54 to 69 inclusive, the diodes 58 to 33 inclusive are conducting and the negative bias voltages it to 84 inclusive are applied to the control elements of the firing electric discharge devices 22 to 21 inclusive" tomaintain these devices non-conducting. However, when a turn on voltage is in duced' in these secondary windings 64 to $9 inelusive, the diodes 58' to 63 inclusive become noncond'ucting thereby releasing t is phase shift voltages-oi transformer windings til to 5d inclusive which then become effective to render the firing electric discharge devices 22 to 2'5 inclusive conducting in accordance with the phase adjustment of the voltages in these windings relative to the anode voltages of the firing electric dis charge devices. I I

Like the power unit just considered, the excit'er 180 electrical degrees out of phase may be pro-" vided for the trigger electric discharge devices 9| to 96 inclusive of the exciter. Each of these trigger electric discharge devices are provided with anodes 91, cathodes 98 and control elements or grids 99. These reference numbers have been applied only to electric discharge device 9| but it is to be understood that corresponding elemerits of the other electric discharge devices 92 to 95 inclusive will be identified by the same reference numbers of the same parts. Each of the trigger electric discharge devices 9| to 96 inclusive have their electrodes enclosed within an envelope which contains, as indicated by the dot, an ionizable medium such as a gas or vapor.

The anode-cathode circuits of trigger electric discharge devices 9|, 93 and 95 are connected across the top half of the secondary windings 88, 89 and so for conduction in response to voltages of one polarity and the anode-cathode circuits of trigger electric discharge devices 92, 94 and 96 are connected across the lower halves of these same transformer secondary windings 88, 89 and 90 for conduction in response to voltages of the opposite polarity.

The cathodes of trigger electric discharge devices 9i and 92 of the A phase section of the exciter are connected together and to the midtap of transformer winding 88 which is grounded. The anode of electric discharge device Si is connected to the upper end terminal of transformer winding 83 through the primary winding I of a transformer l6 and contacts Ifll of an antipolar relay H12 forming part of the sequencer. The anode of electric discharge device 92 is connected to the lower end terminal of transformer 88 through primary winding I03 of a transformer ii and contacts I04 of the anti-polar relay i02. The control element of electric discharge device 9I is connected to its cathode through conductor I85, secondary winding Hi6 of a transformer I01, conductors I08 and I09, an inductance H0, conductor Ill and the lower half of the secondary winding 88 of transformer 82. Since the lower half of transformer Winding 88 supplies a voltage which is 180 electrical degrees out of phase with that supplied by the upper half thereof, it will be seen that electric discharge device 9i will be held non-conductive until a turn-0n voltage is introduced into its control element circuit as will be described below. The control element of electric discharge device 82 is connected to its cathode through a circuit including conductor H2, the secondary winding H3 of a transformer H4, conductor H5, an inductance H6, conductor Ill and the upper half of the secondary winding 88 of transformer 82. For reasons already given in connection with electric discharge device 9|, it is, of course, apparent that electric discharge device 92 is held non-conducting until a turn-on voltage is introduced into its control element circuit as will also be described below. Secondary windings I I8 and H9 of transformers l0 and "H supply respectively turn-on voltages in the control element circuits of trigger electric discharge devices 93 and SM of the B phase.

The cathodes of trigger electric discharge devices 93 and 94 of the B phase sections of the exciter are connected together and to the grounded mid-tap of the secondary winding 89 of transformer B3. The anode of electric discharge device 93 is connected through the primary winding 120 of transformer 12 to one end terminal of the secondary 89 of transformer 83, and the anode of electric discharge device 94 is connected through the primary winding IZI of transformer 13 to the other end terminal of transformer winding 88. Consequently, these electric discharge devices 93 and 9d are connected for anode conduction in response to voltages of opposite polarity. The control element to cathode circuit of electric discharge device 93 is completed through the secondary winding N8 of transformer 10 and the lower half of the secondary 89 or" transformer 83. Thus in the absence of a turn-on voltage in winding H8, electric discharge device 93 is biased off by the voltage of the lower half of transformer winding 89 which is I86 electrical degrees out of phase with the anode voltage of this electric discharge device. The control element to cathode circuit of electric discharge device 9-! is completed through the secondary winding H9 oi transformer H and the upper half or" the secondary winding 89 of transformer 83. This electric discharge device is consequently normally biased to non-conduc tion until a turn-on voltage is introduced in the secondary winding I IQ of transformer l l. Transformer 12 has a secondary winding I22 which introduces a turn-on voltage in the control element circuit of trigger electric discharge device 95 and transformer 73 has a secondary winding I23 which is connected in the control element circuit of trigger electric discharge device 96.

The cathodes of electric discharge devices 95 and 96 of the 0 phase section of the exciter are connected together and to the grounded mid-tap of secondary winding es of transformer 84. The anode of trigger electric discharge device 95 is connected through the primary winding 122 of transformer 14 to one end terminal of the secondary of transformer 34 and the anode of trigger electric discharge device 96 is connected through the primary winding I23 of transformer 15 to the other end terminal of this secondary winding 90. The control element circuit of electric discharge device includes the secondary winding 66 of transformer 12 and the lower half of the secondary winding 90 of transformer 84. The control element circuit of electric discharge device 96 includes the secondary winding 61 of transformers l3 and the upper half of the secondary winding 90 of transformer 84. Consequently when the anode voltages of these electric discharge devices are positive, conduction will not occur until turn-on voltages are supplied by windings I22 and I23 to overcome the electrical degree out of phase voltage otherwise applied to their control elements.

Each of the phase sections of the exciter also contains a phase shift circuit for energizing the primary windings I24, I25 and 525 of phase shift transformers 55, 56 and 57. secondary I24 of transformer 55 is connected to the mid terminal of secondary winding 88 of transformer 82. The upper end terminal of secondary winding 88 of transformer 82 is connected One terminal of the through adjustable resistors I21, I28, and m to the other terminal of the primary I24 of transformer 55 which other terminal primary R4 is also connected through a capacitor I30 to the lower end terminal of secondary 88 of transformer 82. One terminal of primary winding I25 of transformer 56 is connected to the midterminal of the secondary 89 of transformer 83. The upper end terminal of this secondary 89 is connected through adjustable resistors ISI, I 32 and I33 to the other terminal of the secondary I25 of transformer 56 which other terminal of secondary I25 is also connected through a capacitor I34 to the lower end terminal of the second- .ary 89 of transformer 83. One end terminal of the primary I 26 of transformer is connected to the mid-terminal of the secondary 9B of transformer 84. The upper end terminal of this winding 93 is connected through adjustable resistors I 35, I 36 and I 37 to the other terminal of the primary winding I 2.6 of transformer '5'! which other terminal of primary $26 is also connected through a capacitor I 38 to the lower end terminal of the secondary winding so of transformer 84.

Adjustable resistors I22, I3! and I35, in accordance with their setting, provide an adjustment of the welding current supplied by the converter. The respective sliders .of these adjustable resistors are preferably mounted on a common shaft which upon rotation will provide the same heat adjust ment in each of the three phases of the exciter. Adjustable resistors I25, I32 and ltd provide for adjustment of the heat treating or tailing current flow. Their sliders likewise are preferably mounted on a common shaft so that all three phases may have a simultaneous adjustment. Adjustable resistors :29, I33 and I3? are provided for setting the phase shift circuits for full heat, that is for 'initiatingtcurrent flow at the power factor phase angle of the load circuit. The sliders of these rheostats may likewise be tied together for common operation by a common adjusting means. It will be noted that each of the adjustable resistors vide an adjustment of the tailing flow of current, are normally short circuited through contacts 539 of an electromagnetic relay -I at.

Relay !40 has an operating winding Mi, one

terminal of which is connected through conductor i K42, contacts 143 of a relay M4, and conductor I45 to the positive terminal of a direct current source of supply 54E. The other terminal of winding M! of relay hit is connected through a current limiting resistor Ml, arc discharge devices I48 and M9 and conductors c and liiI to the negative terminal of the direct current source of supply I45. Thus when contacts MB of relay I44 are closed and both are discharge devices M8 and I49 become conducting relay winding MI will receive energization from the direct current source of supply M6.

The are discharge devices I '48 and M9 may be referred to respectively as a relay controlling and a switching are discharge device in view of the functions performed thereby. Arc discharge de vice I48 has an anode i 'ii, a cathode 52 and a control element i53. Switching arc discharge device N53 has an anode 54,. a cathode 55 and a control element [59. In each case, theseelectrodes of the respective arc discharge devices are enclosed within an envelope which contains a gas or vapor as is indicated by the dot associated therewith. The control element I55 of are dis- I28, I32 and I35, which procharge device I49 is connected to its cathode I55 through the secondary winding it? of a trans former I58 and a source of bias voltage 5%. Due to the bias voltage I59, arc discharge device E49 is normally held nonconducting until a turn-on voltage is induced in the secondary winding I57 of transformer I 58. When this occurs, switching arc discharge device I43 becomes conducting and connects the cathode 152 of the relay controlling arc discharge device I48 through conductors Ifit and i513 to the negative terminal of the direct current source of supply I46. Arc discharge device =l49 completes its own conducting circuit across the direct current source of supply through a current limiting circuit including resistor I41 which is in shunt to the circuit through relay winding l il, resistor I 4? and the anode-cathode circuit of arc discharge device I43.

Anode-cathode current flow through relay controlling .arc discharge device I is initiatedbt current flow in the firing circuits 28 or .22 of main electric discharge devices .H or I2 of the power unit. Control element r53 of relay controlling arc discharge device I48 is connected to cathode ['52 thereof through conductor I68, a voltage limiting means ISI, conductor I82, negative bias voltage means I59 and the cathode-anode circuit of switching arc discharge device 449. Thus, when arc discharge device MS is conducting and a turn-on voltage appears across the voltage limiting means lii i sumcient to overcome the bias voltage of I53 relay controlling arc discharge device i4il becomes conducting to complete the energizing circuit of the winding I4I of relay I46.

The firing current impulses in firing circuits 20 and 2| of main electric discharge devices H and E2 appearas voltages across resistor It! "through the agency of current transformers P53 and I-64. The primary winding IE5 of current transformer .163 is connected in firing circuit 2% for electric discharge device 1 I, and the primary winding I66 of current transformer {54 is connected in the firing circuit 2| of main electric discharge device it. The secondaries I57 and I68 of these current transformers Hi3 and I S4are connected in parallel with one another across the terminals of the voltage limiting means lfil.

The voltage limiting means IBI is provided for not only protecting the current transformers I63 and I-54 against overvoltages but also for protecting relay controlling arc discharge device I48 from the same overvoltages. These 'overvoltages may occur as the direct result of the great variation in magnitude of the current impulses in the firing circuits 2e and ZI of the main electric'discharge devices II and I2. Protection is provided by using the voltage limiting means in conjunction with current transformers having a substantial voltage regulation resulting in a suitable drooping volt ampere characteristic. The voltage limiting means I6I is preferably formed of a resistance material having a hyperbolic resistance current characteristic productive of a substantially constant voltage across its terminals. Such a resistor is known to the trade as a Thyrite resistor having the physical and operating characteristics disclosed in United States Letters Patent 1,'822,742Karl B. McEachron, granted September 8, 1931..

The sequencer shown at the bottom of Fig. l of the drawing includes electric discharge devices 16.9 to 1175., inclusive. Each .of these electric discharge devices has an anode i126, .a cathlode l1? and a control element H8, which referacvazev ence numerals are applied only to electric discharge I1I in the drawing, it being understood that reference to corresponding electrodes of the other electric discharge devices of this group will be identified by like reference numerals. Each of these electric discharge devices has its electrodes I16 to I18, inclusive, enclosed within an envelope which contains an ionizable medium such as a gas or vapor as has been indicated in the drawing by the dot associated with each device.

It will be noted that the anodes of electric discharge devices IE9 to I12, inclusive, are connected to conductor II1 which is connected to the upper end terminal of the secondary 89 of transformer 82 and that the anodes of electric discharge devices I13 to I15, inclusive, are connected to conductor I I I which is connected to the other end terminal of the secondary winding 88 of transformer 82. The cathodes or" all of these electric discharge devices I69 to I15, inclusive,-

are connected together through conductor I50 which is grounded and consequently connected to the grounded mid-tap terminal of the secondary 88 of transformer 92. Thus, trigger electric discharge device 9| and sequencer electric discharge devices I69 to I12, inclusive, are poled for anode conduction in response to voltages of one polarity, and trigger electric discharge device 92 and sequencer electric discharge devices I13 to I15, inclusive, are connected for anode conduction in response to voltages of the opposite polarity.

Beginning with conduction of electric discharge device I10, electric discharge devices I1I to I15,

inclusive, and 9I and 92 occur in predetermined order during half cycles of voltage of opposite polarity supplied by the secondary winding 88 of transformer 82. This result is secured by completing the anode-cathode circuits of these electric discharge devices through inductances and by connecting the control element of the following electric discharge device to the anode connection with said inductance of a leading electric discharge device. By reason of the inductance in the anode-cathode circuits of each of the leading electric discharge devices, anode-cathode current initiated in one of these devices during a posi tive half cycle of its anode voltage will continue to flow during the first portion of the next negative half cycle, and the voltage drop across the anode-cathode circuit of this electric discharge device due to this carry-over of current flow is used as a control voltage in the control element circuit of the trailing electric discharge device whose anode voltage is positive at the time this current carry-over occurs. This arrangement has been described and claimed in United States Letters Patent 2,514,3'13Maurice E. Bivens, granted November 6, 1951, for Electric Valve Control Circuit.

In the arrangement shown, the anode-cathode circuit of electric discharge device I is com pleted through an inductance I19 and the control element I18 of electric discharge device I13 is connected through a timing circuit I89 to the anode connection of electric discharge device I10 with inductance I19. When electric discharge device I10 is non-conducting and conductor H1 is at positive voltage, capacitor IBI of the timing circuit I80 will be charged through the control element to cathode circuit of electric discharge device I13 so as to present a negative hold-off voltage to the control element of this device. When electric discharge device I10 is rendered conducting, capacitor I8I of the timing circuit I89 discharges through the adjustable resistor I82 of this timing circuit, and after the time delay imposed by the timing circuit, electric discharge device I13 becomes conducting due to the anode carry-over voltage drop across electric discharge device I10.

In like manner, the anode carry-over voltage drop across the anode-cathode circuit of electric discharge device I13 will apply to the control element I18 of sequencer electric discharge device HI and the control element 99 of trigger electric discharge device SI, a voltage which will render these electric discharge devices conducting at the beginning of the next half cycle of voltage following the half cycle of voltage when electric discharge device I 13 was conducting. I

Conduction of electric discharge device I1I will also in like manner produce a trailing conduction of electric discharge devices 92, I14 and I15. Conduction of electric discharge device 92 will immediately follow conduction of electric discharge device I1i, but conduction of electric discharge devices I10 and I15 will occur after the time delay imposed by the timing circuits I33 and I64, respectively, connected in the control element circuits of these electric discharge devices. The time during which welding current flows in a half cycle of the converter load voltage is determined by the setting of timing circuit I83 which consists of a parallel connected capacitor I85 and an adjustable discharge resistance I96. Timing circuit I84 determines the time during which welding current flows and also during which the heat treating or tailing current flows. This timing circuit consists of a capacitor I81 connected in parallel with two adjustable series connected resistors I88 and I89. The setting of resistor I89 determines the tailing current time flow, and the setting of resistor I88 like resistor I96 determines the time for Welding current flow. Preferably the sliders of resistors I86 and I88 are connected on the same shaft so that adjustment of one adjustable resistor secures a like adjustment of the other.

The anode-cathode circuit of electric discharge device I14 is completed through the primary winding I99 of transformer I58. Thus, when electric discharge device I14 becomes conducting, a. voltage is introduced in the control element circuit of arc discharge device I49 which renders this arc discharge device conducting. The anodecathode circuit of electric discharge device I15 is completed through the primary winding I9I of transformer I01. When this electric discharge device I15 becomes conductive, it consequently induces a turn-off voltage in the secondary I06 of transformer I01 which turn-01f voltage is applied to the control element 99 of electric discharge device 9I. The control element I'IB of electric discharge device I12 is connected to the anode connection of electric discharge device I15 with the primary winding I9I of transformer I91. Consequently, electric discharge device I12 conducts in trailing response to conduction of electric discharge device I15. The anode-cathode circuit of electric discharge device I12 is completed through the primary winding I92 of transformer I I4. Consequently, when electric discharge device I12 becomes conducting, a turnofi voltage is introduced in the secondary winding I I3 of transformer I I4 which is applied to the control element 99 of electric discharge device 92 to render it non-conducting at the beginning of the next half cycle of voltage.

Electric discharge devices I69 and I10 are hel accept? firm-conducting but connecting their control .elements through resistors I93 and 1.94, respectively, to conductor i H whose voltage is 180 electrical degrees out of phase with the anode voltage applied to electric discharge devices 59 and $70 from supply conductor HT. These electric :discharge devices are rendered conducting in sequence by a push-button switch 195. When this switch is operated, it "first closes its contacts 1 95 and thereafter its contacts I97. When contacts 456 are closed, the controlelement of electric dischargedevice I69 is connected to its cathode and consequently becomes conducting. In like manner, when cont-acts Isl of the switch are closed, the control element of electric discharge device H8 is connected to its cathode and this electric discharge device then becomes conducting.

Conduction of electric discharge device 1.59 energizes the operating winding of a solenoid valve S. V. which controls the application of fluid pressure to the piston and cylinder arrangement of the welding machine whereby the welding electrodes are brought into engagement with the-work. The operation of electric discharge device H3 in trailing response to conduction of electric discharge device I78 initiates a positive sequence of welding current flow as will appear hereinafter, and the time delay of circuit I8 6 provides a squeeze time within which the welding machine electrodes maybe brought firmly into engagement with the work before welding current flows.

Conduction of electric discharge device Hi9 also energizes the operating winding tilt of relay M4. As previously stated, pickup of this relay completes the anode connections of arc discharge devices I lt and M9 with the plus terminal of the direct current source of supply Mt through its contacts M3. Relay M4 also has a second set of contacts Hill which, when closed, complete the energizing circuit of the operating winding it! of the anti-polar relay me. This energizing circuit is completed from ground through contacts 185, winding Isl, the upper half of the secondary 88 of transformer 82, and thence to ground again. The anti-polar relay I02 may be a single relay, as illustrated, or a plurality of relays which alter nately establish the anode-cathode circuits of electric discharge devices 9i and 92 so that consecutive welding operations may be initiated with voltages of opposite polarity. In the arrangement illustrated, the energization of winding 19? of anti-polar relay I62 operates a ratchet mechanism which alternately makes and breaks circuits through its contacts Hit and I94 which contacts, as previously described, alternately complete and break the anode-cathode circuits through trigger electric discharge devices 9i and 92 of the eXciter. It will be noted by reason of the arrangement illustrated that switches Ill! and HM after operation stay in their operated position until the next energization of the anti-polar relay I62. This result is secured by the star-shaped cam which is rotated a predetermined amount by the ratchet mechanism for each energization of the antipolar relay :52.

As previously stated, the diagram of Fig. 1 of the drawing has been simplified by eliminating certain details any skilled engineer would end-- body in such a system. Thus, the heating circuits for the various control electric discharge devices have not been illustrated nor has the means for imposing a delay in operation of the system until theseheating circuits have been er,- tablished for a predetermined length ol time so that operation of these electric discharge devices an A phase voltage will :of th'eir'respective control will not be premature and result in damage thereto. Other details have been omitted, such as the parallel connected resistor and capacitor in the control element circuits usually provided as a self-biasing means and the grid current limiting resistors also normally included in these control element circuits. Also, the :grid to cathode by pass capacitors usually provided for the control zelectric discharge devices have also been omitted. In the sequencer, the transformer and reactor loading resistances usually employed have-been omitted also. Recitation of these few details will be suggestive of others which are in the same category and which it is to be understood "would be added to the circuit diagram of Fig. l in order to produce those refinements that would be introduced into a commercial apparatus.

The system described above as to physical a-r rangement of parts and their functions will now be further described by setting forth one method of operating-the same.

For convenience in identifying the phase voltages of the three-phase source of sup-ply, the voltage between conductors Li and L2 has been referred to above as the A-pliase voltage, between conductors L2 and L3 as the B phase voltage, and between conductors L3 and LI as the C phase voltage of the three-phase source of supply. A complete phase sequence will thus consist of the A, B and C voltages in their order of occurrence.

Assuming that a three-phase voltage has been applied to the supply conductors LI, L2 and L3,

be applied to the electric discharge devices 169 to 1:75, inclusive, of the sequencer and to trigger electric discharge devices SI and 92 of the A voltage portion of the exciter. By .reason of their connections, electric dischargedevicesfil and I69 to H2, inclusive, may conduct during positive half cycles of the supply voltage whereas electric discharge devices 92 and .133 to 1%, inclusive, may conduct during negative half cycles of the supply voltage. In the B phase portion of the exciter, electric discharge devices 93 and 194 are connected to the B phase voltage so that electric discharge device '93 may conduct during a positive half cycle of supply voltage and electr c discharge device 94 may con- A duct during a negative half cycle of supply voltage. In the 0 phase portion of the exciter, electric discharge devices 95 and 96 are connected to C aphase voltages so that electric discharge device :filimay conduct during positive half cycles of the supply voltage and electric discharge device 9.6 mayoonduct during negative half cycles of the supply voltage. It will also be noted that each (If :the three-phasesections of the power unit is 'under the control of a corresponding phase section of the exciter.

IDur-ing standby when the system is energized, capacitors 'lB-Ji, 1:85 and it! of the timing circuits cf the .sequencer will be charged during positive half :cycles of the supply voltage through rectifying circuits "of the electric discharge devices 173 to H5, inclusive. Meanwhile, the firing electric discharge devices 2.2 "to 2 inclusive, will be biased oil by the bias voltages 15 to 8 l,'inclusive, in the timing branches element circuits. Thus, the phase shift circuits, acting through the heat control branches of these control element cir- 'cuits'are lnellec'tive. Rheostats I29, 833 and It? will be adjusted in accordance with the power factor phase angle of the load current of the welder so that heat control voltages may not be applied to the control elements of the firing electrio discharge devices before the current zero value thus produce transient currents through the electric discharge devices and the load. The rheostats iZl, [3| and $35 of the phase shift circuits will be adjusted in order to secure the desired magnitude of welding current flow during the half cycle of low frequency current supplied by the converter. Ordinarily this adjustment extends over the 3G to 110 period of the phase voltage involved. When oper ating with current flow, however, the range may be no more than between the 30 and 90 points of the volta e wave. Rheostats 528, !32 and res the phase shift circuits will also be adjusted to determine the magnitude of current flow during the heat treating or tailing period which follows immediately after the welding period. It will be noted that each of these last--mentioned rheostats is short-circuited through the contacts of relay Mo whose operating coil t ll is energized in response to current flow the firing circuits as or 2% of the main electric discharge devices i i and i2.

At the start of an operation, electric discharge devices ifiil to i 5, inclusive, are biased off, the anode-cathode circuits of arc discharge devices M8 and M9 are open at contacts Hi3 of relay I44, the anode-cathode circuits of electric discharge devices 9i and 92 are either open at contacts H3: or iii of the anti-polar relay M12 or biased off by 180 out-oi -phase volta es supplied by secondary 33 of transformer and electric discharge devices 93 to Sit, inclusive, are also biased off by voltages 180 out of phase with their re spcctivc anode voltages.

Operation of pushbutton switch I95 first closes its contacts ltd and thereafter its contacts 191. The closure of contacts 595 causes electric discharge device W8 to become conducting to energize the winding of solenoid valve S. V. and to energize the winding I98 of relay M l. The solenoid valve by its operation sup-plies pressure fluid to the piston and cylinder arrangement of the welding machine which then functions to bring the welding electrodes of the Welding machine toward one another and into engagement with the work located between these electrodes. The operation of relay W! closes its contacts I43 and 953. The closure of contacts QM connects the plus terminal of the direct current source M6 to the anodes ltli and let of arc discharge devices Hi3 and iii. The closure of contacts I99 completes the energizing circuit for the operating winding of the anthpolar relay Hi2. Operation of this relay opens its contacts I04 and closes its contact iili. The opening of contacts its opens the anode-cathode circuit of the negative A phase trigger electric discharge device 92, and the closure of contacts till completes the anode-cathode circuit of the positive A phase trigger electric discharge device 9| through the primary winding E! of the trigger circuit transformer '10. Electric discharge device 9i does not conduct however, since its control element voltage is 180 out or phase with its anode voltage due to the supplied by the lower half of the secondary 33 of eizciter transformer 82.

The closure of contacts it! or pushbutton switch IE initiates conduction of electric discharge device E'Efi when its anode goes positive during a positive half cycle of the supply voltage furnished by the upper half of the secondary B8 of exciter transformer 82. Electric discharge device 513, which may conduct during the next following half cycle of supply voltage, does not conduct, however, due to the delay imposed by the timing circuit I89 whose capacitor |8l applies a negative hold-off voltage to the control element of electric discharge device I13 until it has discharged through the adjustable resistor 282 connected in shunt thereto. After timer I has timed out, electric discharge device H3 will conduct during a negative half cycle of the supply voltage in response to the anode carry-over voltage of electric discharge device I10 which, due to the inductance of its anode-cathode circuit, will continue for the first few degrees of the next following half cycle of negative supply voltage occurring after the immediately preceding half cycle of positive supply voltage applied to electric discharge device W3 after timer l8!) has timed out. Timer 1% provides a period during which the electrodes of the welding machine may firmly seat themselves against the work before welding current is applied thereto. This delay period is usually referred to as the squeeze timing period or a weld sequencing control.

Conduction of electric discharge device I13 during a negative half cycle of the supply voltage causes conduction of electric discharge devices ill and 9! at the beginning of the next positive half cycle of supply voltage following the negative half cycle of supply voltage during which electric discharge device I13 was conducting. Thus, conduction of electric discharge device H3 initiates a positive sequence of three-phase supply voltages to the converter as will appear from the following description.

When electric discharge device 9| becomes conducting, it energizes the primary Hit! of trans former iii and induces a turn-on voltage in the secondary windings it and N8 of this transformer. Secondary winding 64 is connected in the timing branch of the control element circuit for firing electric discharge device 22. Its turnon voltage overcomes the bias of Hi also in this circuit to render the diode 58 non-conductive with the result that the heat control branch of the control element circuit of firing electric discharge device 22 takes over and renders it conducting in accordance with the phase shift voltage induced in the secondary winding 49 of the phase shift transformer 55 connected in theAphase shift circuit. Conduction of firing electric discharge device 22 occurs at a time when the anode l3 of main electric discharge device 1 is positive as is its anode 28. A surge or current consequently flows in the firing circuits to of main electric discharge device 1 which renders this device con ducting with the consequent energization of the primary winding 4 of the welding transformer 3 which is thus connected across the A phase supply conductors LI and L2.

The positive turn-on voltage induced in the secondary H8 of transformer Hi is still effective at the time positive B phase voltage is applied to the anode of electric discharge device 93. Consequently, this device will then become conducting energizing the primary winding in of the trigger circuit transformer 12 to induce a turnon voltage in the secondary windings $6 and 66' of this transformer. The turn-on voltage induced. in secondary E6 or" transformer 12 renders firing electric discharge device 24 conducting in response to the phase shift voltage supplied to the secondary 5| of phase shift transformer 56. The positive turn-on voltage induced in secondary winding 65' of transformer 12 is a omas? still effective when positive-C phase voltage is applied to the anode of electric discharge device 95. Consequently, this electric discharge device will then become conducting .to energize primary I22 oftransformer 1-4 which introduces a turn-on voltage into its secondary 08 which causes conduction of firing electric discharge device 26 in accordance with the phase shift setting of the voltage supplied by transformer 51 through its secondary winding 53.

Thus, one complete phase sequenceof supply voltages will be applied to the primaries 4, 5 and '6 of the Welding transformer 3, since like what occurred in the A phase section of the power unit upon conduction of firing electric discharge device 22, conduction of firing electric discharge devices 24 and 25 will apply urges of current through the control elements I5 of main electric discharge devices 9 and II of the Band C phase sections of the power unit, which will cause these devices to conduct in sequence with main electric discharge device 9 taking over conduction from main electric discharge device 1 and main electric discharge device ii taking over conduction from main electric discharge device 0. Conduction of main electric discharge device 9 connects the primary winding 5 of thewelding transformer across supply conductors L2 and L3, and conduction of main electric discharge device II connects the primary winding ,6 of the welding transformer 3 across supply conductors L3 and LI.

This positive sequence of three-phase voltages will repeat itself until trigger electric discharge device -9I i biased off by conduction of electric discharge device I15 of the sequencer. Conduction of electric discharge device I75 energizes the primary winding I9I of transformer I! to introduce a turn-01f voltage in the secondary I05 of this transformer. This turn-off voltage is in opposition of the turn-on voltage supplied by the anode-cathode drop of electric discharge device I13. Both of these electric discharge devices .I I3 and I15 conduct during negative half cycles of the supply voltage, and the turn-off voltage of winding I06 is dominant so-that when electric discharge device I15 is conducting, electric discharge device 9I no longer responds to conduction of electric discharge device I13. It will be noted that winding I 1 5 of transformer I0! has been reversedin the circuit so as to apply the proper polarity of control voltage to secure this turn-off action, that is, transformer I0] is effectivedue to the flux decay voltage induced in its winding I05.

Electric discharge device I I5 conducts after the time interval imposed by timer I84 which times out after the combined time for welding curent flow and tailing current flow has elapsed. Thus, .it is seen that conduction of electric discharge device I15 terminates the positive half cycle of welding and heat treating current ,flow.

In the arrangement illustrated, change-over from welding to tailing current .flow during the output halfzcycle of the converter occurs between the firing of main electric discharge devices II and l and in response to the firing of main elec tric discharge device II. This change-over is secured, by initiating energization of the operating winding Hll of relay I40 in response to the firing of main electric discharge device I I by the pulse of current flow in its firing circuit 20. As previously described, .the energization of operating winding 14! oflrelay I49 is controlled by the conduction of both of the arc discharge devices Band I48} The time .for welding current -flow during the low frequency half cycle of the converter is determined by timer I83 which controls the delay period between conduction of electric discharge device I14 in response to conduction of electric discharge device ITII. When electric discharge device 1.14 becomes conducting after timer I83 has timed out, it energizes the primary winding I90 of transformer I58 inducing a turn-on voltage in the secondary winding I51 thereof. This turn-on voltage overcomes the bias voltage I59 in the control element circuit of arc discharge device I49 causing this are discharge device to become conducting and complete its circuit across the direct current source of supply I46 through conductors I and I42, current limiting resistor I41" and conductors I and I50. Once this are discharge device I49 has become conducting, it will remain conducting until its anode-cathode connection with direct current source of supply I 4.6 is interrupted by the opening of contacts I43 of ,relay I44. When are discharge device I45 becomes conducting, it connects the cathode I52 of arc discharge device I48 to the negative terminal of the source of supply I46 through its anode-cathode circuit and conductors I50 and I50'-. The anode .I5I of arc discharge device I48 is connected to the positive terminal of this direct current source of supply through conductor I45, contacts I 43 of relay I44, conductor I42, the operating winding I II of relay I 40, and current limiting resistor I41. Thus, when ,a pulse of current flows in the firing circuit 20 of main electric discharge device .I I, current transformer I63 having its primaryIl65 in this firing circuit and its secondary IS! in the control element circuit of electric discharge device I48 applies a turnon voltage in the control element circuit of arc discharge device I48 which overcomes bias I59 also in this circuit and causes arc discharge device I48 to become conducting.

When arc discharge device I 48 becomes conducting in addition to the conductivity of arc discharge device I49, relay winding MI is energized and operates relay I40 to open its contacts I39. The opening of these contacts introduces the adjustable resistance of resistors I28, I32 and I3! into theA, B and C phase shift circuits of the exciter. Consequently, when trigger electric discharge device 9I again becomes conducting, firing, electricdischarge device 22 will become conducting at a later point in the positive half cycle of its A phase anode voltage and thus cause main electric discharge device 1 to become conducting at this latter time with a consequent reduction in the flow of current to the primary 4 of the weldingtransformer. This same reduced current flow will also occur in the main electric discharge devices ,9 and I I of the B and 0 phases until the low frequency half cycle is terminated by conduction .of electric discharge device I15, as previously described.

It will be noted that irrespective of the weldingcurrent setting determined by the phase shift circuits .of the exciter, there is a 1 0 electrical degree time interval between conduction of main electric. discharge device II and the subsequent conduction of main electric discharge device I.

This is the time period provided for operation of relay I40 in response to the energization of its operating winding I II. Assuming that the average operating time of a relay I40 made in accord with particular design manufacturing instructions .isj60 electrical degrees of a phase voltage cf'the three-phase source of supply. it will be appreciated that there is a considerable period of time available for variations in its operating time such as may occur from slight variations in its manufacture or as a result of variations in the voltage applied to its operating winding. Thus, by synchronizing the energization of relay I40 to the firing of main electric discharge device II, a longer time interval is provided for pickup of the relays than could be accomplished by other means of synchronization such as, for example, by synchronizing energization of the relay with line voltage. Assuming a favorable operating condition in which the welding current adjustment is between the 30 and 90 points on the anode voltage of the phase being employed, there would only be a period of 60 electrical degrees available for pickup operation of the relay if its operation were synchronized with the line voltage. Of course, if a wider range of adjustment is provided for welding current flow, then still much less time is provided for this pickup operation of the relay when its energization is synchronized with the line voltage.

This feature of applicants system has been made graphic in the voltage diagram of Fig. 2 of the drawing. This shows two consecutive complete phase sequences of the three-phase supply voltage, AI, BI and CI followed by A2, B2 and C2. Assume that timer I83 has been set for zero time delay so that the welding current flow will be supplied for only one complete phase sequence, then electric discharge device I14 becomes conducting at about point a of the diagram at the beginning of a negative half cycle of the AI phase voltage. This renders arc discharge device I49 conducting and prepares the synchronizing circuit for operation in response to conduction of arc discharge device I48.

Assuming a full welding heat setting for the phase shift circuits of the exciter, conduction of the main electric discharge devices will occur at the 30 points of their phase voltages. In the system shown, conduction of C phase current supplies energization to the relay winding I4I of relay I40 in response to the firing impulses in firing circuits 20 or 2| of main electric discharge devices II or l2. As pointed out above, during a positive sequence of voltages, the firing pulse in circuit 20 energizes current transformer I63 to apply a turn-on voltage in the control element circuit of arc discharge device I48 which then conducts and completes the energizing circuit of winding I4I of relay I40. For a positive sequence at full heat setting, relay I40 is thus energized at point I) and must pick up before point d when full heat conduction of the A2 phase will start. This provides a 120 electrical degree operating period for the relay.

Assume a welding heat control adjustment of the exciter over the range of 30 to 110 the anode phase voltages of the main electric discharge devices I to I2, inclusive. Then conduction of main electric discharge device I I o! the 0 phase begins at point D for full heat but may be delayed as late at point 0 for minimum heat. For operation outside the heat adjustment range, a relay whose energization was synchronized with the line voltage would have to pick up within the time cd or 40 electrical degrees. Of course, its energization might be applied at an earlier time by some phase shift circuit arrangement, but pickup must occur within this cd range or else the welding period would be shortened if relay pickup occurred before 0, when operating at this minimum heat setting, or lengthened if 20 relay pickup occurred after d when operating at the maximum heat setting. With applicants ar-' rangement, however, pickup may always occur within a range of 120 electrical degrees.

Thus, if Cl phase conduction starts at b, the following A2 phase conduction starts at d or 120 elecrical degrees later and this same gap is always provided irrespective of the heat setting of the exciter for, if conduction of the CI phase occurred at c, conduction of the A2 phase would occur at e for the same heat setting and thus provide the same gap of 120 electrical degrees for operation of the relay.

For a positive sequence of phase voltages, arc discharge device I49 becomes conducting at point a. of Fig. 2 and are discharge device I48 becomes conducting within the range be depending on the heat setting of the exciter. For a negative sequence of phase voltages, are discharge device I49 becomes conducting at the same point a, but conduction of arc discharge device I48 is delayed 180 electrical degrees by the corresponding delay in the operating time of main electric dis charge device I2 relative to the operating time of main electric discharge device II and occurs within the range fg depending on the welding heat adjustment of the exciter. i

It will also be noted that in applicants arrangement relay I is operated by controlling its energization in response to two non-current momentary signal voltages, one of which is permissive to control in response to the other. Thus, arc discharge device I49 becomes conducting in response to a momentary voltage induced in transformer winding I51 which occurs at a time prior to the application of a momentary voltage in the control element circuit of arc discharge device I48 in response to current flow in the firing circuit 20 of main electric discharge device l I. The relay operating circuit is thus conditioned by a momentary signal voltage for operation thereafter in response to another momentary signal voltage which may vary widely in magnitude due to the voltage limiting arrangement provided in the control element circuit of arc discharge device I48.

With applicants arrangement, it will be noted that during a positive half cycle of current flow supplied by the converter, change-over from a welding value of current to a tailing value of current will occur without interruption in the flow oi. current and that consistency in operation is provided by furnishing a sufficient time interval for relay I40 to operate in accomplishing this result.

To initiate the next welding operation, the operator must release pushbutton I and again close the same. Upon release of pushbutton I95, the system resets itself and when the pushbutton is again closed, the system is again set into operation, but this time with a negative sequence of phase voltages as determined by the anti-polar relay I02. When this relay is again operated, it opens the anode-cathode circuit of trigger electric discharge device 9| and closes the anodecathode circuit of trigger electric discharge device 92. Consequently, control is shifted to firing electric discharge devices 23, 25 and 21 which control main electric discharge devices 8, l0 and I2. Since these main electric discharge devices 8, I0 and I2 are reversely connected between the supply conductors and the load conductors relative to the connections of main electric discharge devices 1, 9 and II, negative sequences of three phase voltages will be supplied to the load.

Since the negativessequenceiof zphase voltages begins liloelectrical degrees :later than a positive sequence, it is necessary tormodify the over-all timing operation ofthe sequencer by extending the timing period by one-half cycle. 'Thus, .insteadof having the-time period xend 'uponaconduction ofelectricrdischarge device I15, ;the timing period ends upon conduction of electric discharge device I 1 2 of the sequencer which operates in trailing-response'to electric discharge device 115- but one-half cycle later. When electric dis- .charge device H2 conducts, it energizes thepr-imar-y winding 195 ofttransformer 15M and induces a turn-oil voltage in its winding :H 3 which is "connected in the control element circuit "of electric discharge device '92. With this added step in the operational sequence, thesystem operates to supply a negative sequence :of voltages to the primary windingsd, J5 .and i of .the welding transformer "in the same manner as above :described for the supply of apositiveasequence of voltages.

It is,of course, obvious that applicants invention is notrlimited to the particularsembodiment thereof illustrated in the drawing and described above. Our invention may be variously modified without departingfromthe spirit and scope of the teachings thereof. Other arrangements will occur to those skilled in the art, and modifications of the circuits employed may :be made in f accordance with particular circumstances such as the type of electr ic discharge devices employed, the employment of equivalent circuits and .the like. We, therefore, aim-:inithe appended claims to cover all :such changes and modifications as fall within the true spirit sandscope of our invention.

Vhat we claim as new and desire to secure -:by Letters Patent ofthe United-States is:

1. Means responsive to stwo non-concurrent momentary signal voltages oneof which is permissive :to control in response to :the other there- 'of, said means including :two are discharge devices each having an anode, a cathode, a control element and a control element circuit connectcurrentsupply conductors, a currentlimiting circuit having terminals connected across said direct current supply conductors in series withthe anode-cathode circuit of one :of said are discharge devices, load conductors connected across said terminals of said current limiting circuit-in series with. the anode-cathode circuit :of the other of said a-rcdischarge devices, bias voltage means connected in the control element circuits of said arc discharge devices :for rendering said devices nonconducting, said bias means havinga voltage less than the voltageof either 'ofsaid signal volt} ages, meansior introducing said oneof said mpmentary signal voltages 'in the control element circuit of said onenf said are discharge devices, and means forintroducing said other of-said momentary signal voltage :;in the control element :circuit of said other of said are .dischargedevice.

2. Control apparatus conditioned by a momentary signal voltage for operation thereafter in response to momentary :signal voltages of wide- 1y varying magnitude, said apparatus comprising two are discharge devices .each having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, direct current supply .conductors, a current .limiting circuit havin ten mlna'ls connected across said direct current sun- A ing its said control element and cathode, direct supply conductors and said ply conductors in series with the anode-cathode circuit :of one of said are discharge :devices, load conductors connected across said terminals .01 said current limiting circuit in series with the anode-cathode circuit of the other of said are discharge devices, bias voltage means connected in the control element circuits of said arc discharge devices .:for rendering said devices nonconducting, said biasmeanshaving :a-voltage less than said respective signal voltages, means for introducing said conditioning momentary signal voltage in the control element circuit .of said one of arc discharge devices, transforming :apparatushaving a drooping voltage characteristic and having inductively coupled primary and secondary windings, means for applying said momentary signal voltages of widely'varied magnitude :to said primary winding of said transforming apparatus, a voltage limiting means having a hyperbolic resistance-current characteristic :productive of a substantially constant voltage .across its terminals, :and means connecting "the terminals .of said volt- ;age limiting means across said secondary'winding of :said transforming apparatus and in the control element circuit of said other of :said are discharge devices.

Apparatus comprising supply conductors, load conductors, controlling :means interconnecting said supply conductors :and said load conductors and having a plurality of operating conditions, a timer, means for initiating operation of said controlling :means under one of its operating conditions and for simultaneously initiatingoperation of said timer, two are discharge devices each having an anode, a cathode, :a con- ;trol element and :a control element circuit connecting its said control element and cathode, direct current supply conductor a currentlimiting circuithaving terminals connecting across said direct current :supply conductors in series with the anode-cathode circuit of one of said are discharge devices, means for changing the operating condition of said controlling means, said means including an energizing circuit connected across said terminals of said current limiting circuit in series with the anode-cathode circuit :of the other of said are discharge devices, means responsive to the timing out operation of said timer for introducing a turnon signal voltage in the control element circuit of said one of said are discharge devices, ,means responsive to an electrical characteristic of said controlling means for introducing a turn- -ion signal voltage in the control element circuit of said other of said are discharge devices, and bias voltage means connected in the con- -trol element circuits of said are discharge devices for rendering said devices non-conducting, said bias means having a voltage less than said respective turn-onsignal voltages.

.4. Apparatus comprising alternating current supply conductors, alternating current load con- ,ductors, controlling means interconnecting said load conductors for current flow of one polarity to said load conductors, said controlling means having a plurality of settings determinative ofthe points in the half cycles of voltage of said supply conductors at which current flows from said supply conductors .to said load conductors, means for initiating operation of said controlling means at one setting thereof, ,a relay having an input ,clrcuit and ;,an output circuit operated by conatrollingthe genergization of said input circuit.

means responsive to the operation of said output circuit of said relay for changing the setting of controlling means, and means responsive after a predetermined number of periods of conduction of said controlling means at said one setting thereof for controlling the energization of said relay input circuit.

5. Apparatus comprising polyphase alternating current supply conductors, polyphase alternating current load conductors, controlling means connecting said supply conductors with said load conductors for current flow of one polarity in a predetermined phase sequence, said controlling means having a plurality of settings determinative oi the points in the half cycles of voltage of said supply conductors at which current flows from said supply conductors to said load conductors, means for initiating operation of said controlling means at one setting thereof, a relay having an input circuit and an output circuit operated by controlling the energization of said input circuit, means responsive to the operation of said output circuit of said relay for changing the setting of said controlling means, and means responsive to the initiation of a current flow of one phase of said phase sequence in said controlling means at said one setting thereof after it has occurred a predetermined number of times for controlling the energization of said relay input circuit.

6. Apparatus comprising polyphase supply conductors, polyphase load conductors, a group of electric discharge devices each having an anode, a cathode, a control element, and a control element circuit connecting its control element and cathode, means connecting said anodes and cathodes of said electric discharge devices in circuit with said supply conductors for applying voltages of the same polarity to said load conductors, a phase shift circuit for each of said electric discharge devices, a relay having an operating winding and pairs of contacts each pair ofwhich is connected in a different one of said phase shift circuits for controlling the phase shift setting thereof in accordance with the closing and opening of said contacts, a timer means initiating operation of said timer and conduction of said electric discharge devices in phase sequence beginning with conduction in one of said electric discharge devices, and means effective after the timing out operation of said timer and responsive to conduction in one of said electric discharge devices for initiating the energization of said relay.

'7. Apparatus comprising alternating current supply conductors, alternating current load conductors, a first electric discharge device having an anode, a cathode, a control element, and a control element circuit connecting its said control element and cathode, meansfor connecting said anode and cathode of said first electric discharge device in circuit with said supply conductors and said load conductors, a relay having contacts and an operating winding whose energization controls the opening and closing of its said contacts, normally inoperative heat control means in said control element circuit of said first electric discharge device, said heat control means including the contacts of said relay for setting in accordance with the open and closed conditions of said contacts difierent times during positive half cycles of anode voltage of said first electric discharge device at which current flow is initiated in its said anode-cathode circuit, means for applying in the control'element circuit 24 of said first electric discharge device a turn-on voltage which renders said heat control means effective in said control element circuit, direct current conductors, a second electric discharge device having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means for connecting said anode and cathode of said second electric discharge device in circuit with said operating winding of said relay across said direct current conductors, and means responsive to current fiow in said anode-cathode circuit of said first electric discharge device for introducing in the control element circuit of said second electric discharge device a voltage which initiates anodecathode conduction of said second electric discharge device.

8. Apparatus comprising alternating current supply conductors, alternating current load conductors, a first electric discharge device having an anode, a cathode, a control element, and a control element circuit connecting its said control element and cathode, means for connecting said anode and cathode of said first electric discharge device in circuit with said supply conductors and said load conductors, a relay having contacts and an operating winding Whose energizationcontrols the opening and closing of its said contacts, normally inefiective heat control means in said control element circuit of said first electric discharge device, said heat control means including the contacts of said relay for setting in accordance with the open and closed conditions of said contacts different times during positive half cycles of anode voltage of said first electric discharge device at which current flow is initiated in its said anode-cathode circuit, when said heat control means is rendered efiective, timing means having a first timing period immediately followed by a second timing period, means for starting said timing means and for applying in the control element circuit of said first electric discharge device a turn-on voltage which renders said heat control means effective in said control element circuit, direct current conductors, a second electric discharge device having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means responsive to operation of said timing means at the end of its said first timing period for connecting said anode and cathode of said second electric discharge device in circuit with said operating winding of said relay across said direct current conductors, means responsive to current flow in the anode-cathode circuit of said first eletric discharge device for introducing in the control element circuit of said second electric discharge device a voltage which initiates anodecathode conduction of said second electric discharge device, and means responsive to operation of said timing means at the'end of its said second timing period for applying in the control element circuit of said first electric discharge device a turn-off voltage which renders said heat control means ineffective in its said control element circult.

9. Apparatus comprising supply conductors, load conductors, a main electric discharge device having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode means for connecting said anode and cathode of said main electric discharge device in circuit with said supply and said load conductors, a firing electric discharge device havingan anode, a cathode, a control ole,-

ment, and a control; element. circuit'connecting itssaid control element and, cathode, normally inoperative phaseshift means: connected in the control element circuitof said firing electric dis charge device, means for introducing a turn-on voltage in the control element circuit ofv said firing electric discharge device which renders said phaseshift means effective; arelay having an operating winding and-a: pair of contacts connected in said phase shifting circuit for controlling: the phase shift: setting: thereof inaccordance with the closed and opened conditions oiisaid contacts, means connecting said anode and cathode of said firing electric discharge devicein the control element circuit of said mainelectric discharge device, and means responsive to thefiow of current in. thecontrol el'ement circuit of saidv main electric discharge devicefor initiating the energization-oi said: winding: of said relay.

10. Apparatus comprising polyphase supplyconductors; polvphase load conductors, a. groupof-main electricdischarge devices each'having an anode, a cathode, a: controlzelement, and-1a control element. circuit connecting its said con-- trol element and cathode; meansconnecting said anodes'and cathodesof said; main electric dis charge: devices in circuit with said supply cornductors for supplying, asequenceof phase voltages of the samepolari-ty to said load conduct ors, firing electric discharge devices each havingan anode, a-cathode, as control elementand a control elementcircuitconnecting its said; control element and. cathode, means for connecting the anode-cathode circuits: of: each ofv saidz firing. electric discharge devices circuit of a different one, of said main electric discharge devices; phase shiftcircuits each: of which is connectedina differentone of said control, element circuits. of said firing electric discharge devices, a-relay having, anoperating wind-'- ing and pairs-of contact each .pair of which is connected in a. different: one of. said: phase shift cir--' cuits for controlling the phase shift settingth'ereof in. accordance with the closing and opening; of

said contacts,means for initiation: conduction of I said firing; electric discharge: devices under: the control of saidzphase: shift circuits and-in: phase" sequence beginning withcon'duction of oneof said firing; electric discharge: devices: and means re:-

sponsive after apredetermined time delay to" the a flow of current in the: control element-circuit of oneof said. main electric discharge devices for initiating; the energization of saidrelay.

11'. Apparatus comprising. alternating; current supply conductors, alternating current load conductors,-. a main; electric discharge: device having: an anode; eat-cathode; a: control element and a con trol element circuit connecting its saidxcontrolelee ment andcathode, ,means for: connectingesaid. an-z ofde and cathode of saidmainelectric discharge device in: circuit; with saidsupply. and-said" load: conductors, a firing electric discharge: device'haw ing. an; anode,v a cathode; v a; controL elementand: a. control element circuitconnecting itsasaid, control. element andicathodmphaseshiitlmeans connected" in the control: element circuit ofi said: firing. 6166:"

tricdischarge device-,..means: fo'rzapplying in; the.-

control, element circuit. of. said firing electric: discharge: device a: bias voltage which renders said phase shift meansineflective for,initiating,v an-- ode-cathode conduction: of. said firingelectric discharge device, means-for'introducing in the con trol-element circuit of said firing electric dis,-

charger device a= turn-on voltage; which overcomes said ia a t se an n Sai 9 1 in a control element means; effective for initiating anode-cathodeconduction of said firing electric discharge device, a relay having anoperating winding and a pair of contacts connected in said phase shift means for controlling its setting in accordance with the closed and opened positions of said con-- tacts, a current transformer having a. primary winding and asecondary winding, means connecting said anode and cathode of said firing electric discharge device inseries with the primary; winding of said current transformer andin saidcontrol element circuitof said main electric discharge-device, apair of arc discharge de vices each having an anode, a cathode, a control element" and a control element circuit connecting its said control element and cathode, direct current supply conductors, means for connecting, theoperating winding of said relay through the anode-cathode circuits of said are discharge devices across-said direct current supply conductors, a voltage limiting means connectedacross said secondary winding, of said current transformer and. in said control element circuit of, one of. said are discharge devices in series withthe anode-cathode circuit of the other of. said arc discharge devices, means. for introducing a turn-on voltage in the control element circuit of said other of said are discharge devices after said main electric discharge. device has been rendered conductiveapredeterminednumber of times, and means inthe control element-circuits-of said are discharge devices: for rendering them non-conducting respectively in the absence of said turn on voltage andta volta-ge of: said voltagelimiting means. 7

12. Apparatus comprising alternating current supply conductors, alternating current. load conductors, a-mainelectric discharge device having ananode, a-cathode, a control element andla control element circuit connecting itssaid control elementand cathode, ameans for connecting said anode and cathodeot said main electric discharge device in circuit with said supply and said load conductors, a firing electric discharge device having an. anode, a-cathode a control element and a con'troIelement circui't'connecting its said-control element and cathode, phase shift means connected-inthe control el ment circuit of said firing electric discharge device, means for. applying in the control element circuit of said firing electric. discharge device a. bias voltage which renders said phase shift means ineffective for initiating anode-cathode conductionof said firing, electricv discharge device, means for introducing in the control'element circuit of said firing electric discharge device a. turn-onvoltage which overcomes said Bias voltage and renders said phase shift means effective for initiating anode-cathode conduction of said firing. electric discharge device, a relay having an. operating winding and a pair of contacts. connected in said phase shift means for controlling its. setting in accordance 2a w d n o said; e Waugh he a o e-V,

cathode circuits of said are discharge devices across said direct current supply conductors, means for connecting said secondary winding of said current transformer in said control element circuit of one of said are discharge devices in series with the anode-cathode circuit of the other of said arc discharge devices, means for introducing a turn-on voltage in the control element circuit of said other of said are discharge devices after said main electric discharge device has been rendered conductive a predetermined number of times, and means in the control element circuits of said are discharge devices for rendering them non-conducting respectively in the absence of said turn-on voltage and a voltage of said voltage limiting means.

13. Apparatus comprising polyphase supply conductors, polyphase load conductors, main electric discharge devices each having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means connecting said anodes and cathodes of said main electric discharge devices in circuit with phase pairs of said supply conductors for supplying current of the same polarity to said load conductors, firing electric dis charge devices equal in number to said main electric discharge devices each having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means for connecting the anodecathode circuit of each of said firing electric discharge devices in the control element circuit of a different one of said main electric discharge devices, a control transformer having a secondary winding and a, primary winding connected in the control element circuit of one of said main electric discharge devices in series with the anode and cathode of said firing electric discharge device in said circuit, phase shift circuits each of which is connected in the control element circuit of a different one of said firing electric discharge devices, a relay having an operating winding and pairs of contacts each pair of which is connected in a different one of said phase shift circuits for controlling its phase shift setting in accordance with the closing and opening of said contacts, direct current supply conductors, a relay controlling electric discharge device having an anode, a cathode, a control element, and a control element circuit connecting its said. control element and cathode, means for connecting the anode of said relay controlling electric discharge device in series circuit with the operating winding of said relay, 9, switching electric discharge device having an anode, a cathode, a controlelement, and a control element circuit connecting its said control element and cathode, means for connecting the anode-cathode circuit of said switching electric discharge device between the cathode of said relay controlling electric discharge device and one of said direct current supply conductors and also in the control element circuit of said'relaycontrolling electric discharge device in series with the secondary winding of said control transformer, a first timer for determining the period of current flow in said main electric discharge devices at one setting of said phase shift circuits, a second timer for determining the period of current flow through said main electric discharge devices at said one setting of said phase shift circuit followed immediately by a different setting thereof, means responsive to the timing out of said first timer for applying a turn-on voltage in the control element circuit ofsaid switching electric discharge device, exciting means for in troducing in the control element circuits of said firing electric discharge devices, control voltages beginning with a voltage of a predetermined phase and continuing in phase sequence thereafter, means for completing and interrupting the anode connection of said relay controlling electric discharge device with the other of said direct current conductors, means for initiating operation of said timers simultaneously with the initiation of operation of said exciting means, and means responsive to the timing out of said second timer for arresting the operation of said exciting means.

14. Apparatus comprising polyphase supply conductors, polyphase load conductors, main electric discharge devices each having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means connecting said anodes and cathodes of said main electric discharge devices in circuit with phase pairs of said supply conductors for supplying current of the same polarity to said load conductors, firing electric discharge devices equal in number to said main electric discharge devices and each having an anode, a cathode, a control element and a con trol element circuit connecting its said control element andcathode, means for connecting the anode-cathode circuit of each of said firing electric discharge devices in the control element circuit of a different one of said main electric discharge devices, a control transformer having a secondary winding and a primary winding connected in the control element circuit of one of said main electric discharge devices and in series with the anode and cathode of said'flring electric discharge device in said circuit, phase shift circuits each of which is connected in the control element circuit of a different one of said firing electric discharge devices, a relay having an operating winding and pairs of contacts each pair of which is connected in a different one of said phase shift circuits for controlling its phase shift setting in accordance with the closing and opening of said contacts, direct current supp conductors, a relay controlling electric discharge device having an anode, a cathode, a control element, and a control element circuit connecting its said control element and cathode, means for connecting the anode of said relay controlling electric discharge device in series circuit with the operating winding of said relay, a switching electric discharge device having an anode, a cathode, a control element, and a control element circuit connecting its said control element and cathode, means for connecting the anodecathode circuit of said switching electric discharge device between the cathode of said relay controlling electric discharge device and one of said direct current supply conductors and also in the control element circuit of said relay controlling electric discharge device in series with the secondary winding of said control transformer, voltage limiting means connected across said secondary winding, a first timer for determining the period of current flow in said main electric discharge devices, a second timer for determining the period of current fiow through said main electric discharge devices at said one setting of said phase shift circuit followed immediately by a different setting thereof, means responsive to the timing out of said first timer for applying a turn-on voltage in the control element circuit of said switching electric discharge device,- exciting means for mtroducing in the control element circuits of said firing electric discharge devices beginning with a voltage of a predetermined phase and continuing in phase sequence thereafter, means for completing and in-- ing, an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means reversely connecting said anodes and cathode of each pair of said main electric discharge devices in circuit with phas pairs of said supply and load conductors, phase pairs of firing electric discharge devices associated with each phase pair of said main electric discharge devices, each of said firing electric discharge devices having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means for connecting the anode-cathode circuit of each of said firing electric discharge devices of a phase pair in the control element circuit of a different one of its associated phase pair of said main electric discharge devices, a pair of control transformers each having a secondary winding and a primary winding connected in said control element circuit of a different one of one phase pair of said main electric discharge devices and in series with said anode and cathode of said associated firing electric discharge device in said circuit, phase shift circuits each of which is connected in the control element circuits of difierent phase pairs of said firing electric discharge devices, a relay having an operating winding and pairs of contacts each pair of which is connected in a different one of said phase shift circuits for controlling its phase shift setting in accordance with the closing and opening of its said contacts, direct current sup ply conductors, a relay controlling electric discharge device having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means for connecting th anode of said relay controlling electric discharge device in series circuit with the operating winding of said relay, a switching electric discharge device having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means for connecting the anode-cathode circuit of said switching electric discharge device between the cathode of said relay control electric discharge device and one of said direct current supply conductors and also in said control element circuit of said relay controlling electric discharge device in series with the parallel connected secondary windings of said control transformers, exciting means for selectively introducing a sequence of control voltages in said control element circuits of each group of said firing electric discharge devices whose anodes and cathodes are connected to supply current of the same polarity from said supply conductors to said load conductors, said control voltages of a sequence beginning with a voltage of one phase and polarity and continuing in phase sequence thereafter with the same polarity, means for alternately: selecting the. operating polarity of said. exciting means and for complet-- ing with each said selection the anode connection of said relay controlling electric discharge device with: the other of said direct current conductors, timing means for establishing afirst timing period followed immediately by a second timing period; means for starting said timing means with a: predetermined phase vcltage at one polarity, means: responsive to: the timing: out op:- eration' of said timing means at the. end: off its: said first timing: period for'applying a; turn-on; voltage in the control element circuit of; said switchingv electric. discharge device, and means responsive to. the timing out operation of said timingmeansat the. end of its saidsecond timing period, for immediately arresting the operation of said exciting means when it is operating at said one polarity and after a half cycle delay when it is operating at said other polarity.

16. Apparatus comprising polyphase supply conductors, polyphase load conductors, phase pairs of main electric discharge devices each having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means reversely connecting said anodes and cathode of each pair of said main electric discharge devices in circuit with phase pairs of said supply and load conductors, phase pairs of firing electric discharge devices associated with each phase pair of said main electric discharge devices, each of said firing electric discharge devices having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means for connecting the anode-cathode circuit of each of said firing electric discharge devices of a phase pair in the control element circuit of a difierent one of its associated phase pair of said main electric discharge devices, a pair of control transformers each having a secondary winding and a primary winding connected in said control element circuit of a different one of one phase pair of said main electric discharge devices and in series with said anode and cathode of said associated firing electric discharge device in said circuit, a voltage limiting means connected across said secondary winding of said control transformers, phase shift circuits each of which is connected in the control element circuits of diiferent phase pairs of said firing electric discharge devices, a relay having an operating winding and pairs of contacts each pair of which is connected in a different one of said phase shift circuits for controlling its phase shift setting in accordance with the closing and opening of its said contacts, direct current supply conductors, a relay control electric discharge device having an anode, a cathode, a controlling element and a control element circuit connecting its said control element and cathode, means for connecting the anode of said relay controlling electric discharge device in series circuit with the operating winding of said relay, a switching electric discharge device having an anode, a cathode, a control element and a control element circuit connecting its said control element and cathode, means for connecting the anode-cathode circuit of said switching electric discharge device between the cathode of said relay controlling electric discharge device and one of said direct current supply conductors and also in said control element circuit of said relay controlling electric discharge device in series with said voltage limiting means, exciting means for selectively introducing a sequence of control voltages in said control element circuits of each group of said firing electric discharge devices whose anodes and cathodes are connected to supply current of the same polarity from said supply conductors to said load conductors, said control voltages of a sequence beginning with a voltage of one phase and polarity and continuing in phase sequence thereafter with the same polarity, means for" alternately selecting the operating polarity of said exciting means and for completing with each said selection the anode connection of said relay controlling electric discharge device. with the other of said direct current conductors, timing means for establishing a first timing period followed immediately by a second timing period, means for starting said timing means with a predetermined phase voltage at one polarity, means responsive to the timing out operation of said timing means at the end of its said first timing period for applying a turn-on voltage in the control element circuit-of said switching electric discharge device, and means responsive to the timing out operation of said timing means at the end of its said second timing period for immediately arresting the operation of said exciting means when it is operating at said one polarity and after a half cycle delay when it is operating at said other polarity.

References Cited in the tile of this patent UNITED STATES PATENTS Number Name Date 2,119,194 Babler May 31, 1938 2,492,730 Bishofi Dec. 27, 1949 

